SEXUAL DIMORPHISM, SEXUAL SELECTION, AND ADAPTATION IN POLYGENIC CHARACTERS

Abstract

Conspicuous sexual dimorphism is a which is maladaptive with respect to nat-feature of many species of higher animals. ural selection. Comparisons within and The genetic basis of variation in metrical between closely related species led Darwin characters, including that in sexual di-to conclude that adult males typically are morphism between families or lines, is more modified than adult females or ju-usually polygenic (Falconer, 1960; Frank-veniles of either sex, but that females have ham, 1968b; Wright, 1968, 1977; Bird and often acquired male characters by "trans-Schaffer, 1972; Ehrman and Parsons, ference." It was difficult for Darwin to 1976). Genetic experiments on mice, birds believe that sex-limitation of characters and Drosophila flies indicate that artificial could evolve by selection, but Fisher selection practiced on a character of one (1958, Ch. 6) outlined how divergent se-sex causes not only a direct response of the lection on the two sexes could accumulate character in the selected sex, but also a genes with different effects in males and correlated response of the homologous females, causing a character at first ex-character, if any, in the opposite sex pressed equally in both sexes to become (Shaklee et al., 1952; Harrison, 1953; sexually dimorphic and finally sex-limited. Korkman, 1957; Becker et al., 1964; Eisen The strength of sexual selection is en-and Legates, 1966; Frankham, 1968a, hanced by a polygamous mating system, 1968b; Eisen and Hanrahan, 1972). Such but the possibility of sexual selection in correlated selective responses are attrib-monogamous systems of mating exists due utable to pleiotropy (and linkage) of genes to male competition for early-breeding fe-affecting the characters of both sexes, that males, and mate choice exercised by these is, correlations between the additive ef-females (Darwin, 1874; Fisher, 1958; fects of genes as expressed in males and O'Donald, 1977). Systems of mating are females. The genetic correlation between often thought to evolve in response to eco-homologous characters of the sexes is often logical pressures (reviewed by Selander, quite high (op. cit.). As will be shown, 1972; Brown, 1975; Emlen and Oring, this greatly restricts the rate of evolution 1977), although mating preferences may of sexual dimorphism relative to that for be self-reinforcing (Fisher, 1958; the average phenotype of the two sexes. O'Donald, 1967, 1977; Lande, unpubl.). Sexual dimorphism may result from Darwin and Fisher described qualita-natural and/or sexual selection. Darwin tive methods by which an observed sexual (1874, Part 2) elucidated how natural se-dimorphism could be attributed mainly to lection operating differently on males and either natural or sexual selection. To as-females arises from their distinctive roles sign natural selection as the primary cause in reproduction, or from competition be-requires ecological observations that males tween the sexes for resources such as food, and females follow different ways of life leading to adaptive sexual dimorphism. and employ the dimorphic character(s) He also reasoned that intrasexual contests adaptively in their distinct modes of sur-for mates and intersexual mating prefer-vival or reproduction. Darwin presented ences exert sexual selection, usually on the several such examples, mostly among the males, producing sexual dimorphism lower classes of animals. Selander (1972) 292